This invention relates to a process for improving the surface of articles made from unfilled or lightly filled liquid crystal polymers (sometimes referred to hereinafter as LCPs).
LCPs are well know to the art. These are usually anisotropic esters of aromatic diols such as, for example, resorcinol, hydroquinone, Bisphenol A, etc. with an aromatic dicarboxylic acid such as, for example, terephthalic and isophthalic acid. Normally, either two different diols or two different dicarboxylic acids are used to make such esters; and frequently a hydroxycarboxylic acid such as, for example 4-hydroxybenzoic acid, is copolymerized with those starting materials. Typical LCPs are described in U.S. Pat. Nos. 4,664,972 to Connolly; 4,242,496 to Jackson et al. and 4,360,658 to Jackson et al.
LCPs are important engineering polymers, characterized by excellent mechanical properties, especially high tensile strength, modulus, and heat deflection temperature, and are readily melt-processable at about 300.degree.-400.degree. C. They are particularly suitable for the manufacture of articles such as various structural/mechanical components and electrical/electronic parts. Typical applications of LCPs include, for example, circuit boards, electronic connectors, and high temperature, under the hood uses for automobiles.
In spite of all these valuable properties, melt fabricated (i.e., extruded and molded) articles made from unfilled and lightly filled LCPs have a serious drawback in that their surface layer is very susceptible to abrasion, even when the abrasive force is very slight. In the context of the present disclosure, the expression "lightly filled" means that the LCP composition contains a small amount, usually not over about 5 percent by weight, of fillers. Such melt fabricated articles are characterized by having a shiny surface. Peeling of the surface layer occurs very readily when it is rubbed or subjected to friction. When the surface of a molded LCP article tends to peel, surface finish such as painting or coating is very sensitive to damage. If an LCP surface is bonded to another surface, the bonded surfaces tend to delaminate. A commonplace problem occurs in electronic applications when the LCP article is a substrate for a circuit board, which must be copper-plated. The copper-plated surface together with the top layer of "peelable" LCP delaminates from the LCP substrate, resulting in a poor mechanical or electrical contact.
It, therefore, has been appreciated for a long time that an improvement of the surfaces of melt fabricated LCP articles was needed, but no process has been disclosed to the art that actually explained and solved the problem.